In order to understand the role of nitrification and denitrification in the accumulation of nitrous oxide (N_2O) in the hypolimnetic water of brackish Lake Nakaumi, the effects of dissolved oxygen (DO) concentration on these activities were investigated by incubation experiments. N_2O-producing activity by nitrification (N_2O_N) increased markedly with decreasing concentration of DO. Low DO (10-30% saturation) induced high N_2O_N. In contrast to nitrification, N_2O-producing activity by denitrification (N_2O_D) decreased with decreasing concentration of DO. Little N_2O was accumulated during denitrification under low-level conditions of DO, because of further reduction of N_2O to N_2.Two strains of N_2O-producing bacteria, Aeromonas sp.and Vibrio sp.were isolated from the sediment in brackish Lake Shinj, Japan. The optimum salinities for growth of Aeromonas sp.and Vibrio sp.were 0-20 and 10-20 psu, respectively. Both strains were halophile favorable for brackish water areas. The effect
… Mores of H_2S on the production and accumulation of N_2O by these strains were studied. N_2O-accumulating activities by Aeromonas sp.and Vibrio sp.were accelerated at 1-5 mgS/L and 1-10 mgS/L of H_2S concentrations, respectively. H_2S was an important factor for N_2O accumulation.A new and simple method for the determination of hydroxylamine in environmental water, such as fresh rivers and lakes using hypochlorite, followed by its gas chromatographic detection, has been developed. A glass vial filled with sample water was sealed by a butyl-rubber stopper and aluminum cap without head-space, and then sodium hypochlorite solution was injected into the vial through a syringe to convert hydroxylamine to nitrous oxide. The head-space in the glass vial was prepared with 99.9% grade N_2 using a gas-tight syringe. The nitrous oxide in the gas-phase was measured by a gas chromatograph with an electron-capture detector. The dissolved nitrous oxide in the liquid-phase was calculated according to the solubility formula. Less